Microwave radiation monitor

ABSTRACT

This portable microwave radiation monitor utilizes an antenna formed in a dual Archimedean spiral. The ellipse ratio of the spiral is minimized by selective placement of resistive means over a portion of the antenna and the power density of the electric field is indicated on a meter connected via diode means to the inner terminals of the antenna.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to instruments for microwave power monitoring andmore particularly, to a portable unit responsive to the power density ofa microwave field.

The expanding use of high frequency, or microwave equipment, has broughtwith it a realization that improper or excess exposure to microwaves,may be injurious. Recognition of this fact has given rise to Governmentregulations and equipment must be carefully designed to avoidpotentially harmful leakage.

Since such electromagnetic radiation is invisible and the human bodydoes not physically "sense" the presence of microwave impingement, it isnecessary to use electronic instruments for purposes of detection andmeasurement. There is a widening need for reliable and economicalmonitoring instruments that can be properly used, even by inexperiencedoperators.

2. Description of the Prior Art

Measuring instruments for microwave power have emerged from earlierdevices placed within wave guides, to sophisticated units and componentswhich measure power density when worn or held within the general area ofa suspected source of radiation. The inventor's U.S. Pat. No. 3,641,439,which issued Feb. 8, 1972, discloses a pioneer probe for sensitivelydetecting microwave radiation with a minimum of field perturbation. Moreeconomical units have also been developed, with the sacrifice ofsensitivity and linearity of measurement. U.S. Pat. No. 4,032,910, forexample, discloses a unit utilizing a diode detector and an alarmoperative when the energy of an incident microwave field is above apredetermined level. To date, neither the latter unit, nor others,furnish the necessary reliability or sensitivity for compliance withGovernmental regulations and recognized health safety standards.

SUMMARY OF THE INVENTION

The present invention is embodied in an inexpensive portable unit whichcan be easily positioned in proximity to suspected sources of microwaveradiation leakage such as microwave ovens, heaters, dryers, medicalequipment, and the like.

An object of the invention is to provide a low cost, reliable, portablemicrowave radiation monitor.

Another object of the invention is to provide a microwave radiationmonitor including components which are physically designed andcoordinated to maximize monitoring effectiveness.

Another object of the invention is to provide a microwave radiationmonitor that will yield a substantially linear response to the powerdensity of the electric field incident thereon.

Still another object of the invention is to provide a portable radiationmonitor which can be conveniently tested to insure that it is in workingorder.

In accordance with one embodiment of the invention, there is provided amicrowave radiation monitor comprising an antenna formed in a dualArchimedean spiral wherein the ellipse ratio of the spiral is minimizedby selective positioning of resistive means in proximity to apredetermined region of the spiral, wherein adjacent turns have currentsof opposite phase when the antenna is oriented in a direction formaximum sensitivity to an electric field. The inner ends of the dualspiral are connected by a Shotkey diode that in turn is connected to amicroammeter.

In accordance with another embodiment of the invention, there isprovided a microwave radiation monitor comprising an antenna formed in adual Archimedean spiral and wherein the detecting diode is characterizedby a capacitance that decreases with increases in reverse voltage acrossthe diode. Inductive reactance means couple the diode to the inner endsof the antenna spiral and exhibit a reactance to resonate with the diodecapacitance when the incident field is of substantially the frequencybeing monitored.

A more thorough understanding of the invention, along with a betterappreciation of the objects and novel features thereof, will beavailable from the following description which is made in conjunctionwith the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an instrument embodying the invention, in proximityto a typical source of potentially hazardous microwave radiation;

FIG. 2 is an enlarged layout of an antenna structure for embodying thefeatures of the invention, showing orientation of its individual parts,the connecting terminals, and the location of resistive means forminimizing the ellipse ratio;

FIG. 3 is a circuit schematic of components used in one embodiment ofthe invention for interconnecting the antenna terminals to anappropriate metering instrument; and

FIG. 4 is an illustration of a resistive lead suitable for use inconnecting the metering instrument to the detecting components used invarious embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As seen in FIG. 1, a portable instrument 10, embodying the features ofthis invention, is shown in relation to the schematic of a microwaveoven 20. Typically, the electric field leakage from such a source iseither vertically or horizontally oriented, as illustrated by vectors E.To monitor the power density of suspected leakage, instrument 10 isbrought into close proximity with the areas of potential radiation.

The instrument 10 comprises a meter 11 interconnected with highresistance leads via a wand 12 to an antenna 13. The antenna is mountedupon the rear planar surface of a spacer element 14 that is composed ofpolyvinyl chloride or other material having substantially free spacecharacteristics. The antenna is formed within a plane, in a dualArchimedean spiral. Resistive means 15 are positioned selectively over aportion of the spiral, as explained hereinafter, in order to minimizeits ellipse ratio.

Meter 11 is mounted upon a handle 16 that is rigidly connected to thewand 12. In order to orient the antenna spirals properly for desiredsensitivity to an emerging electric field, the major axis of the handleis positioned relative to the location of the resistive means 15 (inthis case, approximately at 45°) to effect a sensitivity between theminimum and maximum levels.

The enlarged layout of FIG. 2 represents a seven-turn, two-inch diameterdual spiral forming an antenna that is suitable for measurement ofmicrowave radiation at a frequency of 2,450 MHz. Those skilled in theart are familiar with selection of optimum materials and configurationsfor various frequencies.

The spiral is an Archimedean spiral defined by:

    r=aθ

In this equation, r is the radius from the center to a point on thecurve of the spiral; a is a constant; and θ is the angle of rotation inradians. This spiral has a large ellipse ratio due to its smalldiameter, relative to the wave length being monitored. As an antenna,the spiral's maximum sensitivity to an electric field occurs when animaginery line connecting the ends of the spiral is aligned with theelectric field vector. It has minimum sensitivity when the spiral isrotated 90 degrees from this orientation. When in the maximumsensitivity orientation, the current in the outer turn of the spiral,and in the adjacent turn, is at a maximum at the point on the radiuswhere the electric field vector is tangential to the spiral. Evaluationof the currents in such a spiral reveal that the currents in the outeradjacent turns are in phase. Out-of-phase currents in adjacent turns ofthe spiral occur where the radius is approximately one-half of the wavelength being measured. The segments of the spirals with out-of-phasecurrents in adjacent turns are essentially fixed in spatial relation tothe electric vector of the field being monitored and this patternremains essentially fixed as the spirals are rotated relative to theelectric field vector.

In FIG. 2 the pair of spirals 22,23 terminate in terminals 24,25,respectively. To eliminate or minimize the ellipse effect, a small pieceof resistive means 15, which in one embodiment was a carbon-loadedTeflon film, is positioned over the area of high current when theantenna is oriented for maximum sensitivity. This resistive film isinsulated from the antenna spiral with a suitable thin interveningdielectric. The width of the resistive material determines the angleover which the reduction in sensitivity is accomplished. The radiallength of the resistive material from the periphery toward the center,determines the amount of sensitivity reduction. It is recognized thatthe width of the material also has some effect on the magnitude ofsensitivity reduction.

In a basic instrument, a diode terminates the inner ends of the spirals22,23. A single Shotkey diode may be used, such that the dc currentdeveloped by the diode responsive to an incident field, is carried backby resistive leads to a microampere meter.

FIG. 3 shows a circuit schematic of a further embodiment wherein theantenna terminals 24,25 are connected by an inductance, represented by ashort section of parallel wire transmission line 31,32, to a Shotkeydiode 33. The terminals of the Shotkey diode are in turn connected byresistive leads 36,37 to a suitable meter 40. The diode is depictedschematically as comprising its equivalent component parts, a variablecapacitor 34 which decreases with increase in reverse voltage across thediode, and a diode component 35. It is recognized that such diodes havea square law characteristic, and it is desired to monitor the powerdensity of incident microwaves, as opposed to simply field strength.Since the present instrument does not utilize amplifiers, it does nothave available the compensatory effects that may be developed withamplification.

To minimize field perturbation, the high resistive leads 36,37 connectthe metering instrument 11 to the diode detector; but, this highresistance necessitates a high detected diode voltage with the resultantlinear mode operation. To achieve sufficient output to drive the meterfrom diode 33, without the use of amplifiers, the diode must be operatedin its linear region. If the diode alone is used, and the unit iscalibrated in a continuous wave field and then used to monitor amodulated field, an error in the measurement of power density willresult. This occurs because the meter current will be proportional tothe field strength in volts/meter, whereas it should be proportional tothe square of the field strength in volts squared/meter squared, or tothe power density which is the desired calibration of the meter.

The error can be reduced substantially by connecting diode 33 to theantenna through an inductive reactance, such as a short section ofparallel wire transmission line 31,32. As noted, the capacitance 34 ofthe diode 33 decreases with the application of increasing reversedvoltage. This variation in capacitance is used to tune the resonantcircuit comprising the inductance 31,32 and capacitance 34 as thevoltage increases, thereby enhancing the increase of voltage across thediode 33. For a particular configuration and frequency of 2,450 MHz, theshort section of transmission line was found optimally to be onecentimeter long between the diode 33 and antenna terminals 24,25. Thisprovided the necessary inductance to resonate with capacitance 34.

A resistive lead 36 is illustrated in FIG. 4. Appropriate leads may befabricated from monolithic films of carbon-loaded Teflon. The lead maybe tapered along the major portion of its axis 39 and may have aconstant-width end 38. The extent of taper is not critical. In onepreferred embodiment, the taper extended from a maximum of 0.25 inchesto a minimum of 0.025 inches along a length of six inches. The minimumwidth was then maintained for a length of one inch. The resistivematerial in this embodiment was 0.002 inches thick and exhibited animpedance of 375 ohms per square. The narrow end of the lead wasconnected to the transmission line section 31,32 and thence to theantenna terminals 24,25.

Several additional features useful in optimizing the performance ofembodiments of this invention, are also worthy of note. Inasmuch as theorientation of the spirals for minimum and maximum sensitivity, isrelevant to proper use of this instrument, and these areas ofsensitivity differ in orientation by 90 degrees, it has been founddesirable to orient the longitudinal axis of the meter generally so thatit is 45 degrees with respect to an imaginary line connecting the endsof the spiral. This will tend to make an operator position theinstrument for most effective reading. In this position, the meter willprovide the average of the min-max sensitivity for either vertical orhorizontal polarizations of the incident electric field, and there willbe no difference in calibration for either vertically or horizontallypolarized fields.

Embodiments of this invention also include a simple, inexpensive, andconvenient technique to insure that the meter is in proper workingorder. Test terminals 17 are provided in the handle 16. These terminalsare configured to accept the polarized terminals of a conventional9-volt transistor radio battery. Within the handle the terminals areconnected along one of the resistive leads 36,37, e.g. to conductivestrips 41,42 on lead 36, interconnecting the diode 33 with meter 40.Proper polarity is assured by the conventionally polarized large andsmall terminals. When the battery is inserted in these terminals,current flows through the resistive leads and diode 33, to effect apredetermined test reading on meter 40. If any of the components arebroken or operating improperly, the reading will not be normal and itwill be clear that the instrument would also function improperly ifsubjected to a microwave field.

Particular embodiments of the invention have been shown and described.It will be obvious to those skilled in the art that modifications may bemade in order to reflect particular conditions and to effect particularmeasurements. All such modifications as come within the teachings ofthis disclosure and the scope of the appended claims, are intended to becovered hereby.

What is claimed is:
 1. A microwave radiation monitor comprising: a dualArchimedean spiral antenna defined by r=aθ, wherein r is the radius fromthe center to a point on the curve of the spiral, a is a constant, and θis the angle of rotation in radians; means for orienting said antennawith and responsive to an electric field to generate currents therein,the currents in adjacent turns being in-phase and out-of-phase atpredetermined radial positions of r and θ; resistive means proximate toat least one pair of said adjacent turns and coupled thereto in a regionwhere the currents in the adjacent turns are out-of-phase; and detectingmeans coupled to the inner ends of said spiral antenna operative tomeasure the signal therebetween.
 2. A microwave radiation monitor asdefined in claim 1, wherein said resistive means is positioned where ris substantially equal to one-half of the wavelength of a predeterminedfrequency to be monitored.
 3. A microwave radiation monitor as definedin claim 1, wherein said resistive means comprises a film of resistivematerial separated from said coupled adjacent turns by insulation means.4. A microwave radiation monitor as defined in claim 3, wherein saidresistive means is proximate to a plurality of adjacent turns.
 5. Amicrowave radiation monitor as defined in claim 1, wherein saiddetecting means comprises a diode connected across said inner ends ofthe spiral antenna and a meter connected to measure the current throughsaid diode.
 6. A microwave radiation monitor as defined in claim 5,wherein said meter is connected to said diode by high resistance leads.7. A microwave radiation monitor as defined in claim 6, wherein saidhigh resistance leads are tapered with the narrow end connected to saiddiode.
 8. A microwave radiation monitor as defined in claim 5, whereinsaid diode is characterized by a capacitance that decreases as thereverse voltage across said diode increases, and comprising inductancemeans connecting said diode across said inner ends of the spiralantenna, the reactance of said inductance being selected to resonatewith said capacitance when a predetermined frequency is monitored.
 9. Amicrowave radiation monitor as defined in claim 6, including spacedcontacts along one of said high resistance leads, and means to connect aproperly polarized d-c voltage between said contacts, whereby said meterindicates at least a predetermined current flow when the components arein working order.